A semiconductor memory device such as a DRAM (Dynamic Random Access Memory) includes a sense amplifier per paired bit lines so as to amplify a weak signal read from each memory cell. The sense amplifier needs to raise a potential of one of the paired bit lines to a higher write potential and to reduce that of the other bit line to a lower write potential. Due to this, the sense amplifier is structured so that the higher write potential and the lower write potential are supplied to the sense amplifier when the sense amplifier is activated.
A sensing operation is performed by simultaneously activating a plurality of sense amplifiers. For this reason, relatively high current is applied to a higher-potential drive wiring for supplying the higher write potential and to a lower-potential drive wiring for supplying the lower write potential. As a result, power supply lines that supply the potentials to the higher-potential drive wiring and the lower-potential drive wiring have potential changes, respectively. Such potential changes disadvantageously not only become a noise generation source but also cause a delay in the sensing operation. It is, therefore, desired to suppress the potential changes as much as possible.
The potential change (fluctuation) in the power supply line that supplies the lower potential to the lower-potential drive wiring particularly causes a great delay in the sensing operation. That is, more sense amplifiers available in recent years are configured to not simultaneously supply the higher write potential and the lower write potential but supply the lower write potential in advance to stabilize the sensing operation. Therefore, if the lower write potential is unstable, timing of supplying the higher write potential has to be delayed.
Moreover, some sense amplifiers have been recently configured as overdriven sense amplifiers each of which supplies an overdrive potential higher than the higher write potential to the higher-potential drive wiring in an initial period of the sensing operation. In this case, the lower write potential is normally ground potential, so that the lower-potential drive wiring is not overdriven differently from the higher-potential drive wiring. In this respect, the fluctuation in the lower write potential disadvantageously has great influence on sensing speed.
As described above, the fluctuation in the lower write potential disadvantageously adversely influences the initial period of the sensing operation performed by the sense amplifiers and causes a delay in the sensing operation.